Known optical communications networks incorporate a plurality of add/drop nodes or optical terminals which incorporate optical signal amplifiers. The terminals or nodes are interconnected by optical fibers.
A plurality of lasers can be used as transmitters to inject optical signals in parallel into respective receiving ends of optical fibers using a wavelength division multiplexing (WDM) protocol. Other forms of radiant energy transmitters can also be used.
FIG. 1 illustrates a portion of a known system 10. The add/drop nodes, or terminals 12a,b, include a power amplifier 14a, and a pre-amplifier 14b. Optical signals received at a terminal, for example from an up-stream span or optical fiber, such as 16a, which can incorporate a variable optical attenuator (VOA) 16a′, are amplified at the terminal before being coupled to the next fiber optic span 16b. Such amplifiers, pre-amplifiers or power amplifiers, usually incorporate automatic gain control (AGC) circuitry to provide constant total optical power gain.
Although the AGC circuitry produces substantially constant total optical power gain, the system amplifiers introduce gain variations. With nominally identical power amplifiers, and pre-amplifiers each class of amplifier can be expected to exhibit a substantially identical variable gain profile over a wavelength range of interest, best seen, for example, in FIG. 2.
The variable amplifier gain profile for each pre-amplifier and power amplifier causes the actual per channel gain to vary according thereto. The input power profile will also vary, for example where the network is cross-connected. The overall gain experienced by a light path fluctuates as other light paths are set up and torn down in the network. This gain variation translates into light path power variation. The variation propagates from node to node. For example, with respect to FIG. 2, a channel having a wavelength on the order of 1532 nm will be amplified with maximum gain. On the other hand, a channel having a wavelength on the order of 1538 nm will be amplified with minimum gain in each amplifier, assuming that all amplifiers exhibit a similar profile.
As signals travel through the network, some of them will be amplified with a greater gain than others producing output signal power variations. In addition, the gains experienced by a light path will be altered by the set-up, tear-down or power fluctuations of other overlapping light paths in the network. Such variations are problematic in that the optical receivers have limited input sensitivity ranges. The larger the number of amplifiers that a set of channels passes through the greater will be the output power level variability. Too much input power will overload the respective receiver which in turn leads to increased error rates. This in turn limits the number of amplifiers through which a plurality of channels can pass.
With respect to AGC and variable gain profile, the gain on channel i can be represented as,gib(Pin),where gi is the relative gain factor associated with the non-flatness of the amplifiers and b(Pin) is a “normalizing function” due to the AGC. As explained below, b(Pin) indeed varies with the input power profile Pin.
The amplifier is gain controlled such that the total output power divided by the total input power is a constant, Gtot. Hence,
      G          t      ⁢                          ⁢      o      ⁢                          ⁢      t        =                              ∑          j                ⁢                              g            j                    ⁢                      b            ⁡                          (                              P                                  i                  ⁢                                                                          ⁢                  n                                            )                                ⁢                      P            j                                                ∑          j                ⁢                  P          j                      .  consequently, b(Pin) behaves as,
      b    ⁡          (              P                  i          ⁢                                          ⁢          n                    )        =            G              t        ⁢                                  ⁢        o        ⁢                                  ⁢        t              ⁢                                        ∑            j                    ⁢                      P            j                                                ∑            j                    ⁢                                    g              j                        ⁢                          P              j                                          .      therefore, b(Pin), as well as the gain for each individual wavelength, varies with the input power profile.
The gain experienced by channel i through a span such as 16a can be represented as:
            A      i        =                            g          i          ′                ⁢                  b          ′                ⁢                  g          i          ″                ⁢                  b          ″                    L        ,where b′ and b″ (argument omitted for brevity) depend on the power profile at the input of each perspective amplifier, and L is the total loss (including fiber loss, device loss, and VOA loss) through the span. Substituting the expressions for b′ and b″,
                                          A            i                    =                                                                      G                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                    ′                                ⁢                                  G                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                    ″                                ⁢                                  g                  i                  ′                                ⁢                                  g                  i                  ″                                            L                        ⁢                                                            ∑                  j                                ⁢                                  P                  j                                                                              ∑                  j                                ⁢                                                      g                    j                    ′                                    ⁢                                      P                    j                                                                        ⁢                                                            ∑                  j                                ⁢                                                      g                    j                    ′                                    ⁢                                      b                    ′                                    ⁢                                      P                    j                                                                                                ∑                  j                                ⁢                                                      g                    j                    ″                                    ⁢                                      g                    j                    ′                                    ⁢                                      b                    ′                                    ⁢                                      P                    j                                                                                                                    =                                                                      G                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                    ′                                ⁢                                  G                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                    ″                                ⁢                                  g                  i                  ′                                ⁢                                  g                  i                  ″                                            L                        ⁢                                                            ∑                  j                                ⁢                                  P                  j                                                                              ∑                  j                                ⁢                                                      g                    j                    ″                                    ⁢                                      g                    j                    ′                                    ⁢                                      P                    j                                                                                                                    =                                                    g                i                            ⁢                                                G                                      t                    ⁢                                                                                  ⁢                    o                    ⁢                                                                                  ⁢                    t                                                  L                            ⁢                                                                    ∑                    j                                    ⁢                                      P                    j                                                                                        ∑                    j                                    ⁢                                                            g                      i                                        ⁢                                          P                      j                                                                                            =                                          g                i                            ⁢                                                b                  ⁡                                      (                                          P                                              i                        ⁢                                                                                                  ⁢                        n                                                              )                                                  .                                                           
The above expression shows how the span can be represented by a single amplifier with gain factor g=g′g″. Further, for exemplary purposes only and not limitation, the VOA can be adjusted such that L=Gtot. For a perfectly flat amplifier, every wavelength channel will experience unity gain. For exemplary purposes assume L=Gtot=1.
For a non-flat amplifier, the gain experienced by each channel depends on gi. Furthermore, if the input power profile changes, then the normalizing factor also changes.
For a network that is cross-connected, the input power profile at the beginning of each span is unpredictable. Therefore, the overall gain experienced by a light path fluctuates as other light paths are set up and torn down in the network. The amount of gain variation however is bounded.
The normalizing factor b(Pin) can be upper-bounded by replacing all gi with the minimum value, gmin, in the expression for b(Pin),
                    b        max            ≤              b        ⁡                  (                      P                          i              ⁢                                                          ⁢              n                                )                      ⁢          |                        g          i                =                  g          min                      =                              ∑          j                ⁢                  P          j                                      ∑          j                ⁢                              g            min                    ⁢                      P            j                                =                  1                  g          min                    .      
This maximum is achieved when the input power is dominated by the channel with gain factor gmm. Although the channel associated with gmin is often the weakest channel, it is possible in a cross-connected network to have that channel dominate at the input of an amplifier.
Similarly, the minimum occurs when the input power is dominated by the channel associated with gmax, the channel with the largest gain,
                    b        min            ≥              b        ⁡                  (                      P                          i              ⁢                                                          ⁢              n                                )                      ⁢          |                        g          i                =                  g          max                      =                              ∑          j                ⁢                  P          j                                      ∑          j                ⁢                              g            max                    ⁢                      P            j                                =                  1                  g          max                    .      This implies that the worst case variation in b(Pin) is bounded by the variation in gj, regardless of the network configuration, traffic mix, light path routes, and input power profile.
Where the amplifiers exhibit gain variations as in FIG. 2, variation per span can be expected to be on the order of plus or minus 1 dB. This bound remains in effect even when light paths are dynamically added and dropped at various places in the network. This bound promotes network stability as each light path traverses a finite number of spans. As a result, the power variation at the receiver will also be bounded.
While such power variations may be bounded, they can exceed the input sensitivity of system receivers. In addition, there are times where light paths need to be added to an existing network which can also produce output signals which exceed receiver sensitivity.
There thus continues to be a need for communications networks which can compensate for amplifier gain variations thereby reducing input power fluctuation at the receivers. Preferably such compensation circuitry will be substantially transparent with respect to transmitted messages and will be implementable with minimal impact on manufacturing costs as well as on system reliability.